1. Field of the Invention
The invention relates to an optical fiber devices. More particularly, the invention pertains to polymeric optical fiber devices which have a tunable wavelength response in a polymeric fiber Bragg grating.
2. Technical Background
Optical fibers are key components in modern telecommunications systems. Optical fibers are thin strands of glass or polymer capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. Communication systems now increasingly employ optical fibers which, because of their high speed, low attenuation and wide bandwidth characteristics, can be used for carrying data, video and voice signals concurrently. In essence, an optical fiber is a small diameter waveguide characterized by a core with a first index of refraction surrounded by a cladding having a second lower index of refraction. Light rays which impinge upon the core at an angle less than a critical acceptance angle undergo total internal reflection within the fiber core. These rays are guided along the axis of the fiber with minimum attenuation. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction, or polymers such as acrylates, methacrylates, epoxies or vinyl ethers.
An important extension of these communication systems is the use of wavelength division multiplexing, by which a given wavelength band is segmented into separate wavelengths so that multiple signals can be carried on a single installed line. Modern fiber optic communication systems often have the ability to simultaneously transfer light signals having differing wavelengths over a single optical fiber. A typical optical fiber communications system comprises a source of optical input signals, a length of optical fiber coupled to the source and a receiver for optical signals coupled to the fiber. In multiwavelength systems a plurality of nodes may be provided along the fiber for adding or dropping wavelength channels.
Multiwavelength systems require the use of multiplexers and demultiplexers which are capable of dividing the band into given multiples of different wavelengths which are separate but closely spaced. Adding individual wavelengths to a wideband signal, and extracting a given wavelength from a multi-wavelength signal, require wavelength selective couplers, and this has led to the development of a number of add/drop filters. Optical fiber Bragg gratings are important elements for selectively controlling specific wavelengths of light within an optical fiber. Fiber Bragg gratings are a particularly advantageous structure for differentiating and manipulating optical signals based on their wavelength. Fiber Bragg gratings are often formed by selectively exposing photosensitive fiber to light, thereby creating a permanent refractive-index grating along the core of the fiber. A typical Bragg grating comprises a length of optical fiber including a plurality of perturbations in the index of refraction spaced along the fiber length. These perturbations selectively reflect light in a narrow wavelength range centered around wavelength xcexref, where xcexref=2neffxcex9, and where xcex9 is the periodic spacing between grating elements and neff is the effective refractive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for fiber dispersion.
A wavelength reflection phenomenon is brought about when light is transmitted through an optical fiber having Bragg gratings in its core region. If the wavelength of the light is in conformity with the Bragg condition, the transmitted light is reflected in the grating region of the optical fiber. Such an optical fiber is known as an intra-core fiber grating, which is in fact a wavelength filter or wavelength reflector. Fiber gratings can be widely used in optical fiber communication system and can be used as sensors and as reflective mirrors of laser cavities.
Conventional glass fiber Bragg gratings are conveniently fabricated by providing a silica glass fiber with one or more dopants sensitive to ultraviolet light, such as glass fibers having silica cores doped with germanium oxide, and exposing the fiber at periodic intervals to high intensity ultraviolet light from an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce perturbations in the local index of refraction. The appropriate periodic spacing of perturbations to achieve a conventional grating can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams as is well known in the art.
One difficulty with conventional fiber Bragg gratings is that they filter only a fixed wavelength. Each grating selectively only reflects light in a narrow bandwidth centered around xcexref=2neffxcex9. However in many applications, such as multiplexing, it is desirable to have a tunable grating whose wavelength response can be controllably altered. The applications of the fiber gratings can be significantly broadened if the reflection light spectrum of the fiber gratings is tunable. The filtering condition of the grating can be changed if the length of the Bragg period of the fiber gratings is physically changed or if the effective refractive index of the waveguide is changed. A change in the Bragg period of fiber gratings may be brought about by exerting a tensile or compressive force on the grating, or by winding the grating containing fiber around a piezoelectric ceramic modulated by a voltage source. The effective refractive index of the waveguide can be changed by altering the temperature of the fiber grating region.
The difficultly with conventional glass fiber gratings is their limited tunability range. Attempts have been made to produce tunable glass fiber gratings using a piezoelectric element to strain the grating and thereby change the grating spacing. The response to strain of these glass fibers is typically less than ten nanometers at peak distortion. Because glass fibers have low values for the change in refractive index with temperature changes, the temperature tunability of these devices is also limited. The temperature responsiveness of glass devices over an almost 100xc2x0 C. temperature range is usually less than four nanometers. It would be desirable to produce a fiber grating which can be tuned over a wider wavelength band.
The invention provides a relatively low loss polymeric fiber Bragg grating having a wider effective wavelength tunability band than prior art glass fiber Bragg gratings. While gratings in glass optical fibers have a maximum wavelength dependence of less than 0.04 nanometers per degree C in temperature change, gratings made in polymer waveguides have been found to have a temperature dependence of greater than 0.2 nanometers per degree C. In addition, polymers are capable of undergoing much larger ranges of elastic deformation than glass. As a consequence, changes in wavelength as a result of physical strain of the polymeric fiber Bragg grating are significantly greater than those for a glass fiber Bragg grating. In addition, polymers can be selected for fiber and grating formation which have an exceptionally low loss.
The invention provides an optical fiber comprising a tubular cladding which comprises a first polymeric or glass composition having a first index of refraction, wherein the cladding has a longitudinal axis. A core of a second polymeric composition is within the cladding and extends along and around the longitudinal axis. The second polymeric composition has a second index of refraction which is greater than the index of refraction of the first polymeric composition. A Bragg grating having a plurality of spaced grating elements is formed in the core. The arrangement then has a means for changing the spacing between the grating elements.
The invention also provides a method for forming an optical fiber which comprises forming a hollow tubular cladding which comprises a first polymeric composition or glass having a first index of refraction, wherein the cladding has a longitudinal bore. One then fills the bore with a core of a polymerizable composition through an open end of the hollow tubular cladding. One then partially polymerizes the polymerizable composition to form a second polymeric composition which has a second index of refraction greater than the index of refraction of the first polymeric composition. A Bragg grating having a plurality of spaced grating elements is then formed in the core. Means for changing the spacing between the grating elements is then provided.
The invention also provides a method for forming an optical fiber which comprises forming a hollow tubular cladding which comprises a first polymeric composition comprising a residual amount of a first actinic radiation polymerizable composition. The first polymeric composition has a first index of refraction, and the cladding has a longitudinal bore. One then fills the bore with a core of a second actinic radiation polymerizable composition through an open end of the hollow tubular cladding. One then partially polymerizes the second actinic radiation polymerizable composition to form a second polymeric composition which has a second index of refraction greater than the index of refraction of the first polymeric composition. Simultaneously one then forms a Bragg grating having a plurality of spaced grating elements in the core and an additional Bragg grating having a plurality of spaced grating elements in the cladding which is congruent with the Bragg grating in the core. This is done by exposure of the first polymeric composition and the second polymeric composition to two beam interference pattern of ultraviolet radiation, or exposure to ultraviolet radiation through a phase mask. One then provides means for changing the spacing between the grating elements.
The invention still further provides a method for forming an optical fiber which comprises forming a hollow tubular cladding which comprises a first glass composition having a first index of refraction, said cladding having a longitudinal bore. One then fills the bore with a core of a polymerizable composition through an open end of the hollow tubular cladding, and then partially polymerizes the polymerizable composition. A second polymeric composition is formed which has a second index of refraction greater than the index of refraction of the first glass composition. One then forms a Bragg grating having a plurality of spaced grating elements in the core and provides a means for changing the spacing between the grating elements.